Hard water presents many concerns for domestic and industrial users. Hard water is water that has solids dissolved in the form of metallic compound, and most commonly alkaline earth metal carbonates such as calcium carbonate, and magnesium carbonate. Dissolved iron, manganese, and strontium also contribute to water hardness. When the concentration of these compounds are sufficiently high, the water is called hard. Hard water makes it more difficult for detergents and soaps to perform their cleaning functions, and subsequently requires more soap or detergent to perform the same amount of cleaning. Hard water that has high relative concentrations of iron compounds, concentrations above about 0.2 to 0.3 ppm, also can discolor clothing or other items that the hard water flows over. Hard water also contributes to the buildup of scale in commercial equipment, such as boilers and other industrial equipment, as well as buildup of scale on pipes. Scale is a buildup of mineral compounds on the walls of equipment and pipes. This reduces the efficiency of industrial equipment and reduces the life of pipes and boilers.
The classification of water hardness, as measured by the concentration of calcium carbonate is generally as listed in Table 1.
TABLE 1Classification of water hardnessClassificationmg/l or ppmgrains/gallon or gpgSoft  0-17.10-1Slightly hard17.1-60    1-3.5Moderately hard 60-1203.5-7  Hard120-180  7-10.5Very hard>180>10.5
Water hardness is based on ion-chemistry in ground waters, and is relatively stable over time. From the data of wells monitored by the US Geological Survey, a mean hardness was found to be about 217 ppm. From the monitoring of the wells, there are large portions of the United States that have moderately hard to very hard water. While this is not a health problem, but is merely a consequence of the nature of the environment, it also presents problems for many people and businesses that need softened water for uses such as domestic cleaning purposes, or industrial processes where the metallic compounds cause problems with equipment or processes.
Currently, most water softening involves the use of ion exchange resins, wherein the resins are periodically recharged with a salt that creates a soluble compound. Typically, the alkaline earth metal cations, calcium and magnesium, are exchanged with sodium which is supplied in the form of salt in a water softener using an ion exchange resin. The water softener must be periodically resupplied with salt, which is often in the form of large blocks of salt, or a supply of salt pellets that are placed in a container from which the water softener can recharge. Designing a water softener that does not require salt, saves the expense of buying salt, and the expense and time of periodically obtaining the salt and recharging the water softener.